The paper, coming out of the paper machine, is in many cases, passed through a calender in which the sheet of paper is acted upon with pressure. At the same time, in many cases, heat is supplied since one of the rollers forming the gap is heated. The surface of the sheet of paper is smoothed by the combination of heat and pressure.
The required operating speeds of the machines are constantly increasing. At the same time, a higher surface quality for the paper is also required. In order to satisfy both requirements, more heat is being supplied to the rollers. For this purpose, high heat outputs are required, with 20 kW per meter of roller length often used. However, only a very small portion of the heat energy is transferred directly from the heated roller to the paper sheet. The bulk of the heat energy radiates into the environment and is lost. This causes not only a deterioration of the working conditions in the surroundings of the calender, but also results in a considerable waste of energy. Moreover, in many cases, it is not possible to transfer sufficient energy to the paper sheet.
EP 0 501 035 A1 discloses a heat-protection hood for heated rollers, particularly for high-speed rollers in paper machines. The heated roller is constructed as the top roller of an arrangement with at least two rollers. The sheet of material is passed through the gap between the heated roller and a counter-roller. A hood surrounds the heated roller over a portion of its extent. A separating screen is disposed between the hood and the roller which forms two gaps. At the ends of the gaps, air deflectors are disposed, which deflect the air entrained in an inner gap adjacent the heated roller into the outer gap. By these means, heat losses due to convection are reduced. By dividing the heat-protection hood axially, it is also possible to influence the axial temperature profile of the rollers. In this arrangement, the hood is often inverted only over the roller and it covers only one end roller. Therefore, considerable amounts of energy are still lost.
DE 41 17 596 C1 shows a calender for treating sheet material, particularly a sheet of paper, with a heated roller, which is disposed at one end of a stack of rollers. In order to avoid heat losses, the heated roller is provided with a cover, formed by a sheet pulled over the extent of the roller and held at a distance from the roller by an air cushion. In order to avoid fluctuations in paper smoothness during the operation of the calender, it should be possible to install and dismantle this cover quickly.
DE 21 47 021 A1 shows a method and an apparatus for drying a sheet of moist material. Here several heated rollers are disposed in a housing, which has inlet and outlet openings for the sheet material. However, the sheet material is not passed through roller gaps, but is exposed to the atmosphere existing in the housing. In this arrangement, the moisture in the sheet material evaporates. The heated vapor, or steam, atmosphere is collected in a heat exchanger and the heat recovered from the steam is used to heat the heated roller.
DE 26 15 634 A1 discloses a method and an apparatus for treating sheet material, particularly a paper sheet, the material of which is dried down to the hygroscopic range. In this arrangement, the sheet material is passed over suction rollers which aspirate a steam atmosphere from a space enclosed by a housing through the material. It is intended that the sheet material be dried at those places which are too moist and moistened at those places which are too dry. The space, surrounded by the housing, is supplied with steam for this purpose.